I have an 03 Discovery and tow a 3500lb Gross Pop Up Camping Trailer. I would like to upgrade my tires to 255/70 -16's. Would my discovery have issues towing my trailer with this tire size change?

 

You should have no problems what so ever. These trucks are rated for 7700lbs tow capacity. The US manual says 5500 in high range and 7700 in low range, but the UK manual doesn't specify if I remember correctly. Either way, you're perfectly fine.

 

You have the 4.6 liter, so it should do better than my 4.0 liter, but mine sucks towing my 2 place snowmobile trailer. I have 255/60-18 tires, which are about the same diameter as the 255/70-16's (30") you are looking at. When I pull my trailer up most grades, it won't stay locked up in 3rd gear and is always shifting into 2nd gear and revving the snot out of the engine. The stock size will work better while towing a trailer. I wish it were a manual instead of the automatic so I could hold it in the gear that works best.

 

My handbook states 3500 kgs (EU laws and all that cr@p) so he should be safe provided the tyres and towbar are correctly rated.